This invention relates to metal oxide varistors and, more particularly, to a metal oxide varistor with an improved structure which provides more desirable electrical properties and to a method of making the improved varistor.
In general, the current flowing between two spaced points is directly proportional to the potential difference between those points. For most known substances, current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohm's law to be the resistance. There are, however, a few substances which exhibit nonlinear resistance. Some devices, such as metal oxide varistors, utilize these substances and require resort to the following equation (1) to quantitatively relate current and voltage: EQU I=(V/C).sup..alpha. ( 1)
where V is the voltage applied to the device, I is the current flowing through the device, C is a constant and .alpha. is an exponent greater than 1. Inasmuch as the value of .alpha. determines the degree of nonlinearity exhibited by the device, it is generally desired that .alpha. be relatively high. .alpha. is calculated according to the following equation (2): EQU .alpha.=log.sub.10 (I.sub.2 /I.sub.1)/log.sub.10 (V.sub.2 /V.sub.1) (2)
where V.sub.1 and V.sub.2 are the device voltages at given currents I.sub.1 and I.sub.2, respectively.
At very low voltages and very high voltages, metal oxide varistors deviate from the characteristics expressed by equation (1) and approach linear resistance characteristics. However, for a very broad useful voltage range the response of metal oxide varistors is as expressed by equation (1).
The values of C and .alpha. can be varied by changing the varistor formulation and the manufacturing process.
Another useful varistor characteristic is the varistor voltage which can be defined as the voltage across the device when a given current is flowing through it. It is common to measure varistor voltage at a current of one milliampere and subsequent reference to varistor voltage shall be for voltage so measured.
Still another varistor characteristic of use to circuit designers considering varistors is the leakage current. Realizing that varistors are normally exposed to line voltage during use, it is clear that some current will constantly flow therethrough. This leakage current is wasted and thus it is desirable to minimize it. Also, the leakage current can cause joule heating in the varistor, possibly causing premature device aging or characteristic changes. consequently, it is generally desired to keep the leakage current as low as possible.
The foregoing is, of course, well known in the prior art.
Metal oxide varistors are usually manufactured by mixing a plurality of additives with a powered metal oxide. Usually zinc oxide is used, but it should be realized that other base oxides such as those of titanium, germanium, iron, cobalt, nickel, and vanadium can be used. Typically, four to twelve additives are employed, yet together they comprise only a small portion of the end product, for example, less than five to ten mole percent. In some instances the additives comprise less than one mole percent. The types and amounts of additives employed vary with the properties sought in the varistor. The additives are usually metals, metal oxides, or metal flourides. Copious literature describes metal oxide varistors utilizing various addiombinations. For example, see U.S. Pat. Nos. 3,642,664, 3,663,458, and 3,687,871, or my copending patent application Ser. No. 467,274, filed Nov. 19, 1973, now U.S. Pat. No. 3,878,602 titled, "Metal Oxide Varistor With Discrete Bodies Of Metallic Material Therein And Method For The Manufacture Thereof." A portion of the metal oxide and additive mixture is then pressed into a body of a desired shape and size. Next, the body is sintered for an appropriate time at a suitable temperature as is well known in the prior art. Sintering causes the necessary reactions among the additives and the metal oxide and fuses the mixture into a coherent pellet. Leads are then attached and the device is encapsulated by conventional methods.
Varistors manufactured by the aforementioned techniques function well in most applications. However, as is the case with most electronic components, certain particularly demanding applications require a device with improved characteristics. Specifically, some applications require a varistor with a higher alpha which will clamp more effectively. Other applications require a varistor that will consume less power when in its standby mode. That is, they require a varistor with a lower leakage current.
It is, therefore, an object of this invention to provide a metal oxide varistor with improved electrical properties such as an increased alpha and a lower leakage current, and to provide a method for manufacturing the varistor.